Current Solar Activity

Fig. 1: The Sun imaged 2005 July 03 at 06h 40m UT. A 1/1000 sec. exposure ISO 100, Baader filter, 100mm OG.

Fig. 2: A composite photograph from images of the Sun taken on 2005 July 02 (06h 50m UT) and the photo from fig. 1.

As reported earlier, this spring and summer (so far) has proved one of the most disappointing from an observational point of view since I commenced astronomical work in Orkney over thirty years ago. One hopes for clear nights close to summer solstice in the expectation of seeing noctilucent clouds (see archives). This is especially important close to minimal solar activity.

Despite the poor weather conditions some work on the Sun has been possible. There have been short periods with no visible sunspots but in the main the Sun has been quite active.

The apparent daily movement of spots is mostly a result of the Sun's rotation. This rotation varies with the solar latitude being much faster towards the Sun's equator. Spots tend to appear in a relatively narrow band either side of the equator. Spots have not been observed close to the poles and so they cannot be used to determine the Sun's rotation across the full range of the globe.

The apparent path taken by a spot or group of spots is close to a straight line during early June and early December. At other times the path is either concave or convex to the solar equator. The movement of spots during a twenty-four hour period can be appreciated from the composite photograph, fig.2. The images from photographs taken on 2nd and 3rd July have been superimposed. Fig. 1 shows the single image from July 3rd.

Twilight

Nautical Twilight (when the Sun's upper limb is between 12 and 6 arc degrees below the horizon) ends on 29 July for the latitude of Orkney.

Summer Full Moon

The enlarged full moon of June 22 had considerable media coverage. Here in Orkney we had one of many cloudy nights.

There were two contributory factors to the Moon appearing larger on that night. First, full moon occurred close to perigee when in fact the Moon was at its closest to Earth. Second, the Moon had an extreme southerly declination of a little over 29°. From Orkney the Moon's altitude at meridian passage was fractionally over 2°, or four times its own apparent diameter. The Moon was only above the horizon for 2½ hours and so throughout this period it would have been very close to the horizon.

It was mentioned several times that the Moon "appeared larger than normal". One has to ask what is normal in this context? Any full moon will appear larger when it is close to the horizon at rising or setting. This is due to the well established psychological effect that the human brain associates the appearance of the Moon with objects observed horizontally by the eye. A similar phenomenon occurs when one looks down from a height on objects below compared to looking upward at the same objects.

This suggests that posture is as important, if not more important, than other factors. The simple, if rather undignified, experiment of looking at things upside down from between one's legs will illustrate the point.

One has to be cautious when making comparisons. Pain threshold varies considerably from person to person. Some people are unable to distinguish certain colours, others are what is called tone deaf. I therefore submit that one person's "normal" Moon may be another's not so normal!

JV 03/07/05